Large Scale Earthquake Simulation of a Hybrid Lead Rubber Isolation System Designed under Nuclear Seismicity Considerations
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The design of the isolation system was constrained by the experimental setup. The light axial loads on the system, which are not representative of a nuclear facility, necessitated the use of a hybrid system of bearings and flat sliders, known as cross linear (CL) bearings. The CL bearings support beneath some of the columns without contributing to the system base shear, so that the desired isolation period could be provided at the target displacement. Additionally, the CL bearings provided substantial resistance against the tensile demands generated by overturning as a result of the light axial loads. Thus, the suitability of a hybrid isolation system for nuclear power plant was evaluated as part of the test program. A numerical simulation model was developed for the isolation system and the structure. The lead-rubber bearings were modeled with a bilinear force-displacement relation. Due to the amplitude dependence of the bearing response, the parameters of the bilinear model were calibrated independently for each simulation. Using the calibrated model, the predicted displacement demand of the isolators was within 10% of the observed experimental displacement. For typical XY (horizontal only) input excitation, the horizontal accelerations at the roof level were reduced by a factor of 10 relative to the fixed-base building. Under 3D input excitation which included vertical shaking, the vertical accelerations of all floor slabs were amplified by a factor of 4-6 relative to the input vertical excitation, but the vertical amplification factor was essentially the same for the isolated and the fixed-base building; thus the isolation system did not increase vertical acceleration relative to the fixed-base building. Horizontal-vertical coupling was detected in the vibration modes of the structure (both fixed-base and base-isolated). The coupling was attributed partially to a mass irregularity. The majority of the coupling effects were replicated by a well-crafted numerical simulation model that accounted for slab-frame interaction and refined distribution of mass over the floor system. The design of the base isolation system and structure should consider and accommodate these predictable horizontal-vertical coupling effects.
Report No. CCEER-13-09